In the operation of electric steel melting furnaces, carbon or graphite electrodes are employed to conduct electric current to the metal through an electric arc which is struck between the end of the electrode and the metal to be melted. The heat that is developed by the electric arc not only melts the metal but also gradually consumes the electrode. It is therefore necessary to continuously feed the electrode into the furnace in order to maintain the electric arc. Eventually, as the electrode is consumed, a new electrode section is added by joining it to the end of the old electrode section.
A common method of joining the two electrode sections together is by use of a threaded nipple. The nipple is screwed into a correspondingly threaded socket provided in the end faces of the two electrode sections. Although this type of electrode joint has been used for many years now, it has not proven to be entirely satisfactory and is still the source of many problems. For example, there is a tendency for the electrode sections to occassionally unscrew and create loose joints when subjected to severe conditions of temperature, vibration and mechanical and thermal shock. The occurrence of loose electrode joints can be a major problem resulting in high electrical resistance and increased electrode consumption. In extreme cases, complete loss of an electrode section into the furnace may occur due to unscrewing and loosening of the electrode joint.
Attempts have been made to solve these problems by introducing an electrically conductive, carbonizable cement such as coal tar pitch between the nipple and both electrode sections in order to permanently bond the electrode joint. For example, in U.S. Pat. No. 2,828,162 to H. V. Johnson et al, a pitch filled reservoir is located within each end of the electrode nipple and channels or passageways are provided to distribute the liquid pitch upon heating to the threaded area around the nipple. The pitch fills the void spaces between the nipple and socket threads. Upon further heating, the pitch cokes or carbonizes, solidly cementing the joint and providing a permanent bond between the electrode sections. The bond is mechanically strong and of lower electrical resistance than an uncemented joint.
U.S. Pat. No. 2,836,294 to H. V. Johnson et al discloses a pitch cartridge adapted to be inserted inside a reservoir provided in the electrode nipple. The cartridge includes an outer fibrous, thermally insulating jacket which is consumable at a temperature well above the melting point of the pitch. The jacket defers the time at which the pitch melts and avoids a premature distribution of the liquid pitch before the electrode joint has been fully formed.
Despite these attempts to prevent loosening of the electrode joint, the problem still prevails and is most evident when the electrode is subjected to severe conditions. It has been found, for example, that high stresses that are created by thermal cycling of the electrode column can break the pitch coke bond between the nipple and socket threads, leaving only the abrasiveness of the coke residue on the threads to resist unscrewing and loosening of the joint. Moreover, the degree and frequency of vibration of some furnace operations can result in an unscrewing torque sufficient to overcome the resistance provided by the pitch coke, rendering the pitch filled reservoir joint ineffective. Also, before the liquid pitch cokes out on the nipple and socket threads, the frictional force between the threads is slightly reduced, which can initiate high column unscrewing and eventual loosening of the joint under certain operating conditions.
It has been proposed to improve the pitch coke bond between the electrode nipple and socket threads by employing a foaming agent along with the carbonizable pitch. The foaming agent upon heating forces the pitch out from the reservoir and into contact with the threads on both the nipple and electrode socket.
U.S. Pat. No. 4,007,324 to R. W. Wallouch discloses an electrode nipple having a reservoir containing a heat-foamable, thermoplastic material comprising particulate pitch and a lignin sulfonate-based binder. The high temperatures provided at the electrode joint foam the composition and cause it to expand into adjacent thread spaces at the joint.
However, the foaming agents so far employed with carbonizable cement in electrode joints have not proven to be altogether successful. First of all, the degree of swelling or expansion of the liquid pitch upon heating to the threaded area of the joint has actually been somewhat limited. Furthermore, the particular foaming agents employed have had little or no effect on the coking reaction and have not decreased the time required to implement the bond between the nipple and socket threads.
An important object of the present invention is therefore to improve the effectiveness of pitch filled reservoir electrode joints.
A more specific object of the present invention is to increase the resistance of such joints against unscrewing and loosening during use in an electric arc furnace.
Another object of the present invention is to decrease the time required to implement the joint unscrewing resistance and thereby lessen the chances of the joint becoming loosened due to the reduction of frictional forces on the threads by the presence of liquid pitch.
Still another object of the present invention is to minimize the concentration of mechanical stresses on the nipple and socket threads by a more uniform pitch distribution.